As conventional optical biosensors using the surface plasmon resonance method, there is disclosed, for example, the one having two or more minute openings provided in a metallic film at predetermined intervals (see patent document 1). According to this optical biosensor, surface plasmon waves propagated on an upper side of the metallic film are resonantly phase-aligned, increasing amplitude, and then concentrated on the minute openings. For this reason, a major part of the transmitted light occurring at the lower side of the minute openings comes from components originated from the surface plasmon waves propagated through the upper side of the metallic film, rather than the components of light irradiated directly to the minute openings and transmitted therethrough. Accordingly, detecting the transmitted light occurring at the lower side of the minute openings makes it possible to detect the components of the surface plasmon waves transmitted at the upper side of the metallic film. As the components of the surface plasmon waves contain information about a substance to be detected present at the upper side of the metallic film, the substance can be detected by detecting the transmitted light occurring at the lower side of the minute openings.    Patent document 1 Japanese Un-examined application publication No. 2005-16963